Spearthrower wrote:How could we intentionally do it, or how it could happen regardless of our intent?

For the former, population growth rates are already in decline and have been for decades. So from a most banal method, simply by having less babies. If each generation produces only one offspring per couple, we would see very rapid declines in total population. Alternatively, as discussed in another thread, if tyranny were not an objection then policies could be formulated to disincentivize having more than 1 child. Whatever the case, if sufficient numbers of people were convinced through awareness of our impact on the Earth and its systems that our population is harming our ability to thrive on an individual and species-wide level, then we could intentionally lower our numbers consistently over decades. Is it likely? I am not sure as I don't have a lot of faith in humanity as a whole to act rationally, and there's always the problem of the tragedy of the commons motivating acquisitive behavior, but it's not completely unthinkable given some recent events where humans have chosen to forgo some immediate benefits in place of long term sustainability, i.e. the Montreal Protocol.

For the latter, there are numerous ways...

So the question was how a population of 7 billion might become a population of 2 billion, with options both for an intentional decline in population, and an unintentional decline in population. The former, I suggested might occur if people were convinced that our population is having a detrimental impact on the environment that they might seek to have only one child per couple per generation, and that this would eventually have the effect the question was asked to explain.

But now we're talking about China's One Child Policy (and what might have happened in an alternate universe had it not been or things that might have been some consequence of it), which the sharp-eyed among us might note a) didn't lower population at all (it grew by half a billion) because it wasn't really a one-child policy in anything other than name b) wasn't motivated by ecological concerns.

That is, of course, all by-the-by because the answer still stands as true. If the majority of humanity elected to have one child per couple per generation then population numbers would quickly stabilize then drop rapidly. The actual effect would be exponential... up to a point! Thus the question is answered: this is one (of many) ways that our population could go from 7 to 2 billion and no murders are required.